Electrochemical CO2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor

Abstract

Electrochemical CO 2 reduction reaction (CO 2 RR) to liquid fuels is currently challenged by low product concentrations, as well as their mixture with traditional liquid electrolytes, such as KHCO 3 solution. Here we report an all-solid-state electrochemical CO 2 RR system for continuous generation of high-purity and high-concentration formic acid vapors and solutions. The cathode and anode were separated by a porous solid electrolyte (PSE) layer, where electrochemically generated formate and proton were recombined to form molecular formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities~450 mA cm −2), selectivity (maximal Faradaic efficiency~97%), and stability (100 hours) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concentrations of pure formic acid solutions (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.

Boosting thermo-photocatalytic CO2 conversion activity by using photosynthesis-inspired electron-proton-transfer mediators

“…67 and −0.87 V (vs. RHE) were used in previous reports 43 – 45 . Furthermore, the direct reduction of CO 2 by the electrochemical approach is not a perfect technology because it still suffers from low product concentrations in the mixture of traditional liquid electrolytes 46 . As a result, the product cannot be directly used without further purification.…”

“…As a result, the product cannot be directly used without further purification. However, the purification of the low concentration liquid fuel in electrolytes not only compromises energy efficiency, but also is a technical challenge 46 .…”

Enabling storage and utilization of low-carbon electricity: power to formic acid

“…Recently, there is an increasing number of reports of CO2-to-FA conversion utilizing organic solvents and solid electrolytes to achieve higher CO2 solubility, to limit HER activity and temperature resistance, and to allow easy downstream separation of products. [183][184][185][186] In the case of aqueous electrolytes, the use of bicarbonate (HCO3 -) and carbonate (CO3 2-) anions as the source of CO2 can enhance the eCO2RR by increasing the local CO2 concentration. 187,188 On the other hand, Li + , Na + , K + , H + and NH4 + cations were found to favor eCO2RR where KHCO3 solution was the commonly adopted aqueous electrolyte and could be widely used for various electrodes including Cu, Sn, Hg, Pb, carbon, gas diffusion electrodes (GDEs) etc.…”

“…184 (b) Schematic illustration of all solid-state reactor containing solid electrolyte for eCO2RR to pure FA. 185,186 Since eCO2RR is often carried out in the aqueous solution to facilitate ion conduction between electrodes, it is inconvenient and energy-intensive to separate a liquid product like FA from dissolved salts. To overcome this difficulty, very recently, Wang and co-workers sought to employ a porous solid-state electrolyte (PSE/SSE) in an eCO2RR cell to produce neat FA (Fig.…”

“…With further advancement in the field of PSE, an allsolid-state eCO2RR system was introduced for continuous generation of high-purity and high-concentration FA vapors and solutions which can be efficiently removed in the form of vapors via inert N2 gas flowing through the PSE layer. 186 As a result, concentrated FA could be achieved without sacrificing much selectivity compared with the deionized water flow design. Coupling with a high activity (jFA ~450 mA/cm 2 ), selectivity (FEmax ~97%) and stability (100 h) grain boundary-enriched Bi catalyst, an ultra-high concentrations of pure FA solutions (up to ~100 wt%), condensed from generated vapors via flexible tuning of the carrier gas stream through PSE.…”